Oh, No, no, no, It has effected on default network!
    # Prepare environment and record current net_state_dict
    backup = network_xml.NetworkXML.new_all_networks_dict(virsh_instance)
    backup_state = virsh_instance.net_state_dict()
    logging.debug("Backed up network(s): %s", backup_state)

    try:
        default_xml = backup['default']
    except (KeyError, AttributeError):
        raise error.TestNAError("Test requires default network to exist")

    # Prepare default property for test
    # Transeint network can not be set autostart
    # So confirm persistent is true for test
    default_xml['persistent'] = True
    netname = "default"
    netuuid = default_xml.uuid
    # Set network 'default' to inactive
    default_xml['active'] = False

    currents = network_xml.NetworkXML.new_all_networks_dict(virsh_instance)
    current_state = virsh_instance.net_state_dict()
    logging.debug("Current network(s): %s", current_state)

The output is:
before set active to False(just did "default_xml['active'] = False"):
Backed up network(s): {'default': {'active': True, 'persistent': True, 'autostart': True}}
after:
Current network(s): {'default': {'active': False, 'persistent': True, 'autostart': True}}

The default network has been set to inactive. :S